The invention relates to a milling installation for the generation of meal-fine products, in particular for the generation of cement raw meal and/or of cement from cement clinkers, with a comminuting device for the comminution of the raw materials, a sifter for the separating-off of the fine material generated in the comminution from the still-present coarse material and with a fine material separating device for the separating-off of the fine material from the sifting air of the sifter.
As a comminuting device in known milling installations for the comminution for meal fineness there are predominantly used impact hammer millers, tube mills, or a roller press, when raw material is to be milled into the fineness range of cement raw meal or of cement and when a relatively great throughput is desired.
Downstream of these comminuting devices there is then ordinarily engaged a sifter, in which the comminuted raw material is sifted at the desired grain size, with the aid of sifting air, i.e., is separated. The sifters used there are, according to the problem definition, dynamic or static sifters. In the sifter the still too coarse comminution material is separated from the sufficiently comminuted fine material, conveyed out of the sifter over a sluice and then returned to the comminuting device. The fine material leaves the sifter with the sifting air and is thereupon separated from the sifting air in a corresponding device. This device is ordinarily a further sifter with a downstream-engaged dedusting device.
In order to keep the dedusting gas amounts as low as possible, it is a known practice there to return a major portion of the purified sifting air to the sifter in a closed circuit.
In the so-called milling drying in which, with the aid of the sifting air, moist raw substances are dried during their comminution and/or their sifting, hot gases are used as sifting air such as, for example, furnace exhaust gas, cooler exhaust gas or hot gases of a hot-gas generator. In these cases then the amount of the returned sifting air for thermal reasonsxe2x80x94fresh hot gas is needed for the dryingxe2x80x94is decreased or altogether no sifting air is returned any longer.
Especially for the generation of cement raw meal and for the comminution of cement clinkers in known milling installations in recent time a so-called V-sifter has been used which, as a static sifter without driven components, is distinguished by a low specific energy requirement.
Thus in EP 0650763 BI a circulating milling installation is described in which raw material is milled with the aid of a roller press to cement raw meal fineness, in which process the sifter used in the milling installation is a static cascade sifter designated as a V-sifter, with two sifting zone-boundary walls enclosed by the shaft-form sifter-housing, and that form a sifting zone between them and are also being traversed by the sifting air in transverse flow, which (walls) have guide plates in cascade or Venetian blind manner inclined obliquely downward in the direction to the discharge opening for the sifted-out coarse fraction, in which system the two guide plate walls, and therewith the interlying sifting zone are arranged to lie obliquely at an angle deviating from the vertical.
The coarse fraction separated off in this V-sifter is led back to the roller press, while the fine material fraction together with the sifting air enters into a cyclone dust separator, which is connected with a second-circulating mill-installation with tube mill and sifter of its own, upon which there follows a dedusting device.
In a publication of the firm of KHD Humboldt Wedag AG (xe2x80x9cImprovements in raw meal and cement generation by V-sifter and roller press,xe2x80x9d Zement-Kalk-Gips No. 3, March 1997, pp. 140-147) there is presented a further example of an application for the use of the V-sifter in the cement raw meal and cement generation. In this example, in the cement raw meal generation the limestone comminuted in a roller press is fed to a V-sifter, in which there occurs a separating of the coarse material from the generated fine material. While the coarse material is returned to the roller press, the fine material with the sifter air passes into an after-engaged cyclone sifter, in the after-engaged cyclone separators of which the finished cement raw meal is separated off in the desired fineness.
Although with the static V-sifter there is available a simply constructed device with low wear characteristics and a low specific energy requirement, still, for example in the milling drying for the generation of a cement raw meal for the furnace feed, the usual specific energy requirement for the milling and dedusting blower remains at 4 to 9 kWh per ton of generated raw meal, which is still relatively high.
It is the problem of the invention, therefore, to further develop known milling installations with an integrated V-sifter so that without cost-intensive constructive measures there is obtained a clear lowering of the specific energy requirement for the entire milling process.
By means of the invention, by connecting the V-sifter directly to a fine material separating device, preferably to an electro-filter, to form a homogeneous component, without using the otherwise usual pipelines, there is yielded first of all a clear reduction of the specific energy requirement for the entire milling installation, and namely primarily by the elimination of the flow resistance otherwise caused by the pipelines, for the sifting air, which otherwise must be compensated for by an increase in electric drive performance for the blower.
If according to the invention an electro-filter is used, a tube-form filter is likewise possible, then moreover, additional dedusting of the sifting air is no longer necessary, whereby likewise a saving in specific energy is made possible.
Simultaneously with the lowering of the energy costs, additional installation costs are advantageously saved by the elimination of the otherwise usual pipelines and the elimination of the otherwise necessary dedusting device downstream from the separation of the fine material.
By causing the constructive dimensions and attuning of the V-sifter to correspond to those of the fine-material separating device, it becomes possible, for example in the cement raw meal generation in which the meal is dried to maintain the operation of the cyclone heat exchanger furnace, of the milling installation and of the dedusting with only one suction blower.
So that the V-sifter can be directly connected with the fine-material separating device without interposed pipelines, it is necessary to arrange the outlet opening of the V-sifter for the sifting air laden with fine material not as otherwise usual at the upper end of the sifting housing, but laterally on the sifting housing, so that the sifting air at least can also be fed laterally into the fine-material separating device.